1. Field of the Invention
The present invention relates to an electro-photographic image forming apparatus. More particularly, the invention relates to an image forming apparatus for forming images by the electro-photographic process using copiers and printers.
2. Description of the Related Art
Many electrographic copiers and printers form images on one side of a recording material such as recording paper. Now, however, what is called the double-sided image forming apparatus, which is capable of forming images on both sides of a sheet for environmental protection and savings of natural resources, has been commercialized. The double-sided image forming apparatus prints images on a first side and then on the other side, utilizing a paper turn-over mechanism that turns over the sheet of which one side has been printed and a re-feeder mechanism that feeds the sheet again.
FIG. 1 is a diagram illustrating an example of the structure of the prior art electro-photographic laser beam printer. This laser beam printer has a sheet turn-over unit and a re-feeder unit near the center of the printer 100, and has a detachable transfer unit D for double-sided printing in the body. A paper cassette 101 that houses sheets of paper P is located at the bottom of the body. Sheets P are transported by a transport roller 108 to a process cartridge 112 via a pickup roller 104, a feeder roller 105 and a retard roller 106 that feed paper, separating sheets P one by one. Upstream of the process cartridge 112 are a pre-resist sensor 110 that detects the sheets P and resist rollers 109 that transport the sheets P synchronously.
The process cartridge 112 is detachably attached to the body and forms an electrostatic latent image with laser light from a scanner 111 on a photosensitive drum 1 working as the image carrier. A visible image or toner image is produced by developing this latent image. The scanner 111 is generally comprised of a laser unit 129 that emits laser light, a polygon mirror 130 that scans the laser light from the laser unit 129 on the photosensitive drum 1, a polygon motor 131, an image formation lens assembly 132 and a return mirror 133. The process cartridge 112 is equipped with the photosensitive drum 1, a charger 2, a developer 134 and a cleaner 6 that are all needed in common electro-photography. Conventionally, the charger 2 is usually a non-contact type corona charger that charges the photosensitive drum 1 surface by providing corona produced by high-voltage applied to a thin corona discharge wire. In recent years, however, contact-type chargers have been most preferably used because of their advantages of lower pressure process, less ozone emission and lower cost. This is a method of, for example, contacting a roller charger material (hereinafter, a roller charger) to the surface of the photosensitive drum 1 and charging the photosensitive drum 1 by applying voltage to this roller charger 2. Although voltage applied to the roller charger 2 may be DC voltage alone, charging becomes uniform if AC voltage is additionally applied to repeat a plus/minus discharge alternatively. By exposing the uniformly charged photosensitive drum 1 to laser light using the scanner 111, the desired latent image is formed thereon and this latent image is transformed into a toner image by the developer 134.
A development bias is applied to the development roller constituting the developer 134. As the bias voltage for development, only DC voltage is applied when the development roller 134 contacts the photosensitive drum 1, while AC voltage is added to DC voltage during non-contact operation. The toner image on the photosensitive drum 1 is transferred to a sheet P by a transfer roller 113.
Downstream of the process cartridge 112 a fixer F affixes the toner image transferred to a sheet P by applying heat and pressure thereto. The fixer F is generally comprised of a fixer roller 117, a heater 116 that heats the fixer roller 117, a pressure roller 118 and a temperature sensor 140, such as a thermistor. The pressure roller 118 is pressed against the fixer roller 117 by a spring unit (not shown). Downstream of the fixer F are fixer exit rollers 139 and a fixer unit sensor 119 that detects the passage of a sheet P.
Downstream of the fixer exit rollers 139, the transport path is branched and a flapper 120 decides the way of paper transport. In usual single-sided printing, a sheet P is conveyed to the outside of the body by the output rollers 122, while for double-sided printing it is sent to the transport unit D.
The transport unit D for double-sided printing has a sheet turn-over unit equipped with reverse rollers 123 and a reverse sensor 124, and a re-feeder unit equipped with a D-cut roller 125, a sensor 126 and transport rollers 127.
The transport path is branched upstream of the reverse rollers 123, and the reverse sensor 124 is installed near the branching point. A sheet P is stopped in the position where the end of the sheet P has traveled a prescribed distance passing the reverse sensor 124, and then sent to the re-feeder unit by reverse rotation of the reverse rollers 123.
When the turn-over unit sensor 126 has detected the passage of the sheet P, the transport rollers 127 convey sheet P to the transport roller 108 again for re-feeding. Later, the sheet P passes the resist rollers 109 again, and the transfer roller 113 conducts image formation on the other side of the sheet P. Then the sheet P is guided by the flapper 120 to output rollers 122 for output after toner is fixed by the fixer F.
In this type of image forming apparatus, the number of sheets waiting in the transport path in the sheet turn-over mechanism and re-feeder mechanism is determined according to sheet sizes, and their printing sequence is optimized for efficient double-sided printing (for example, as discussed in Japanese Patent Application Laid-open No. 2002-091102). If a large number of sheets are to be printed double-sided, their printing sequence is changed so that the number of sheets waiting in the transport path in the sheet turn-over mechanism and re-feeder mechanism is maximized according to sheet sizes. Such changes of printing sequence are conducted by altering the page sequence based on printing information that is sent from a PC, for example, and stored in the memory of the printer.
However, when the memory capacity in the printer is small, it cannot hold the printing information of many pages and thus the printing sequence cannot be changed. When the memory capacity is small, the sheet is turned over after its first side is printed and then re-fed for printing on the other side (rear face). Each of two or more sheets is printed in this manner. Then, instead of plural sheets, only one sheet is held in the transport path of the sheet turn-over mechanism and the re-feeder mechanism.
Regardless of memory capacity, when only one sheet is printed double-sided, the sheet is turned over after one side is printed and re-fed for printing on the other side (rear face). In addition, when a double-sided copy is made by scanning a document with a scanner, printing is done while the document is being scanned. Since the page sequence cannot be changed in this case, it is repeated in many cases to turn over the sheet after one side is printed and then re-feed it for printing on the other side, when two or more document pages are scanned for double-sided copying.
When the sheet is turned over after one side is printed and then re-fed for printing on the other side and therefore the transport path in the sheet turn-over mechanism and the re-feeder mechanism holds only one sheet at a time, it takes time to turn over and re-feed the paper. Then the power to the charger for the electro-photographic process is suspended, or the heater for fixing is deactivated to prevent the image carrier from wearing and unnecessary heater operation (for example, as discussed in Japanese Patent Application Laid-open No. 8-320642).
However, in such a double-sided image forming apparatus, there will be a significant difference in the rotation time of the photosensitive drum per sheet between continuous double-sided printing and double-sided printing on only one sheet.
FIG. 2 is a timing chart for continuous double-sided printing in the prior art image forming apparatus, and it illustrates the timing for continuous 4-sheet double-sided printing. FIG. 3 is a timing chart for one-sheet double-sided printing in the prior art image forming apparatus.
In general, after AC voltage and DC voltage for charging are raised to prescribed values, DC high-voltage is applied as the bias voltage for development in the pre-rotation process, and then AC high-voltage is applied in the printing process as the bias voltage for development. Transfer high-voltage is applied when a sheet P passes the transfer unit. During the interval of sheet printing, the AC high-voltage for development is lowered and the transfer high-voltage is also lowered to a level for the interval. When the last page is printed, the post-rotation process starts, and the transfer high-voltage, DC high-voltage for development, DC high-voltage for charging and AC high-voltage for charging are lowered in this order.
In FIG. 2, when a first side of the first sheet is printed and the sheet has reached the turn-over point, a first side of the second sheet is printed. When the first sheet has reached the transport unit in the turn-over unit and the second sheet has reached the turn-over point, a first side of the third sheet is printed, and then the second side of the first sheet, a first side of the fourth sheet and the second side of the second sheet are printed sequentially. When the second side of the third sheet and the second side of the fourth sheet are printed in a row, the double-sided printing on four sheets is over.
Referring now to FIG. 2, because printing is completed in a short time in continuous double-sided printing, the interval period of time per sheet does not much affect the life of the photosensitive drum 1. The life is as long as that of the drum used in continuous single-sided printing.
On the other hand, when double-sided printing is repeated for each single sheet, the steps of printing on a first side, paper interval, and printing on the second side are repeated, as shown in FIG. 3. Such operation is seen when the memory does not have a capacity large enough to store the image data of plural pages or when an image forming apparatus equipped with a read scanner conducts double-sided copying. During the time interval between printing on a first side and printing on the other side, namely, the period of time from the turn-over of a sheet P to its re-feeding, the photosensitive drum 1 keeps rotation. Because usually it takes as much time as printing two or three pages to turn over sheet P and re-feed it, the life of the photosensitive drum 1 becomes equally shorter.
Image forming apparatuses are expected to run faster and faster. Thus if the next feed process is started after the feeding of each previous sheet is completed, the feeding speed itself must be raised. Otherwise, even if the feeding speed is raised, there will be a limit to throughput.
To solve such problems, printing data is stored in a printing data reservation memory, and as soon as the printing requirements are met paper is fed for printing based on the data stored in the memory, in order to feed not only the next sheet but also further latter sheets at a time (hereinafter, preliminary feeding; for example, as discussed in Japanese Patent Application Laid-open Nos. 2002-046876, 2001-192132, 2001-088406 and 2001-088370). By virtue of this improvement, throughput can be easily maximized without raising the paper feeding speed too much or raising print cost, even when the transport path for recording sheets is rather long.
In many printers, a single driving source (motor) is used to rotate the image carrier and transport rollers for lower cost. The motor is directly connected to the driver of the image carrier, while its connection to transport rollers is switched by a clutch. In the image forming apparatus of such structure, the sheet is turned over after its first side is printed and then re-fed for printing on the other side. Then a single sheet is held for double-sided printing in the transport path in the sheet turn-over mechanism and the re-feeder mechanism. If the abovementioned preliminary feeding is adopted in this system to maximize throughput, the following problems arise.
If a single sheet is to be printed double-sided, it is possible to stop the rotation of the image carrier by suspending high-voltage for electro-photography while the one-side printed sheet is turned over and fed again. However, in the case of continuous double-sided printing of plural sheets, the transport rollers must be kept rotating for preliminary feeding of the subsequent sheets, while the one-side printed sheet is turned over and fed again. Since the image carrier shares the driving source with the transport rollers, its rotation cannot be stopped during preliminary feeding.
As a result, throughput can be maximized with no increased cost, but such a problem results that the image carrier wears fast and comes to the end of its life early because it keeps rotating and receives a high-voltage while the one-side printed sheet is turned over and re-fed.
In cases other than double-sided printing, a similar problem will arise when the paper interval is long in usual single-sided printing.